Firewire Jitter Worries ???

[mark4man]

All these damn newfangled Firewire interfaces, man !!!

This from the Seneschal site:

Since FireWire is a packet switched bus, there's no synchronous clock carried along to convey timing. And, there are myriad sources of jitter within the 1394 transport protocol, the main source being that you have a free running oscillator in every node on the bus. Says Julian Dunn,“The IEEE 1394 format uses asynchronous clocks at each node. The interaction of these clocks with each other and with the sample (word) clock generates jitter.”

I'm about to upgrade my studio's converters & was bustin' my butt to stay w/ PCI (or go to AES/EBU w/ an AES > PCI interface)...'cause If no less than Julian Dunn is worried about jitter, then I am to.

But...say the A/D conversion is taking place inside an outboard AI or breakout box...& that breakout box is in turn feeding the firewire buss the converted digital signal (thru the DAW software & onto the hard drive)...I guess I'm saying I don't understand what the worry is...'cause the main concern with jitter is \"conversion\" jitter, right? If the audio is converted to digital outside (& prior to) the box, then esentially you're using the firewire bus to transfer the data to the HD inside the workstation.

Is that also a concern?

mark4man

 

[T-Dogg]

Someone feel free to correct me if I'm wrong, but from what I get of this whole jitter thing, I think you'd be in good shape, provided the outboard AD converter is set to use it's own internal clock as the sync reference during the tracking stage. This would then mean you'd be going digitally into the soundcard interface... So say you're using ADAT lightpipe; the clock is embedded in the signal to the soundcard, then passed through firewire's looser clock structure. Regardless, those 1's and 0's will make their way to the hard drive in the right order and stored \"jitter free\"... But your realtime monitoring will likely be jittery, as would playback because firewire has \"retimed\" your signal, unless:

1, The ideal situation -- your outboard DA converter can \"resync\" the clock for playback (alot of nicer, newer ones can do this, I think the Benchmark stuff for example...)

2, You use word clock to sync everything -- the soundcard, DA, and AD together. Even here, the ideal would be to set the AD's clock to internal and have the other units slave to it for recording... Then, for mixing set the DA to internal and have the soundcard sync to it... This would give you near jitter-free playback and recording (at least to the resolution of your AD DA clocks)... But you may need to repatch the wordclock connections between mixing and recording to make it happen...

 

[Cabbage]

I would say jitter is not an issue with Firewire. The sender and receiver have independent clocks, and one is not controlled by the other (like they are with spdif, adat, word clock...). Syncing the two with word clock would still be possible, but this would only solve very significant drifts, not jitter.

I think Bob Katz is addressing this in his book \"Mastering Audio\", but someone has borrowed it from me, so I cannot check it.

Petter

 

[Tarekith]

Actually in his book, Bob is very much against the current firewire implentation because of the jitter issues. He mentions it specifically.

 

[Tarekith]

The more I think about this though, the less I think jitter is an issue for this use. I think Bob was referring to firewire audio interfaces, where jitter via firewire is going to be directly translated to the D/A clock. For something like a UAD firewire device, the signal is 100% digital round trip, and jitter will not apply. There's no D/A or A/D stage.

Unless I'm thinking about this wrong...

 

[jcat]

The more I think about this though, the less I think jitter is an issue for this use. I think Bob was referring to firewire audio interfaces, where jitter via firewire is going to be directly translated to the D/A clock. For something like a UAD firewire device, the signal is 100% digital round trip, and jitter will not apply. There's no D/A or A/D stage.

Unless I'm thinking about this wrong...

Sounds right to me, jitter AFAIK can only be introduced during the A/D or D/A stages.

Think of ethernet as well, there is no clock for the cards to sync to while running FX Teleport etc, but no jitter will be introduced, just compensated delays.



Cheers,

jcat